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Old 5th November 2009, 07:20 PM   #111
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Latest update on my 26 pre. Replaced the plate resistors with 200H custom built plate chokes by Jack Elliano@Electra Print. Worth every penny. Greatly improved the sound stage as well as it gave more authoritative and precise top to bottom. Thanking to Jack for his quality piece of work.
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Old 5th November 2009, 09:44 PM   #112
Richard is offline Richard  Australia
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45,

Thanks once again, I'll give transformers a shot hopefully this weekend.

Fingerx,

I'm using a 1.2 ohm 5W wire wound. Just divide V ref by the current of the filament. 1.25V/1.05A = 1.2 ohms.

I thought I might have to use the Ronan regulator, but the CRC filter with big caps works fine.

Cheers,

Rich
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Old 6th November 2009, 05:17 AM   #113
fingerx is offline fingerx  Hong Kong
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Thanks Rich, 1.25V got it
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Old 8th November 2009, 09:20 AM   #114
CFT is offline CFT  United States
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I remember somewhere I read that someone combined a + and a - filament voltage regulators and yielded tiny hum. Could this because the signal path was somewhat isolated from the power supply?

So another thought here...how about in the filament regulator, one puts a 1083 current source, the filament and another 1083 current source in series, would that creat any enhancement to hum isolation from ps?
Thanks for your comment!
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Old 8th November 2009, 09:40 AM   #115
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Use a gyrator in the +ve path, and a transistor current source in the negative path.

This really knocks out the hum, and gives wonderful sound, too.

schematic:

New DHT heater

Adding the isolating transformer is very effective against mains noise.
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Old 8th November 2009, 01:37 PM   #116
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I can tell you that Rod Coleman's filament supply design is the best I've ever heard. Well worth a build. I use a LM1084 ccs for convenience, but for a serious build, Rod's circuit is better.

andy
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Old 8th November 2009, 04:45 PM   #117
CFT is offline CFT  United States
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Great Rod, so that nice circuit orginated from you!

I gathered at the end of that post, Thorsten suggested something like using the LT1085/1033 pair.

Could anyone come up with that circuit? Would it simply be that just using 1085 ccs in the + path to filament while 1033 ccs put in place at the - path? So a current source and a current sink is born?
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Old 8th November 2009, 05:08 PM   #118
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Quote:
Originally Posted by CFT View Post
Great Rod, so that nice circuit orginated from you!

I gathered at the end of that post, Thorsten suggested something like using the LT1085/1033 pair.

Could anyone come up with that circuit? Would it simply be that just using 1085 ccs in the + path to filament while 1033 ccs put in place at the - path? So a current source and a current sink is born?
Sadly, that would give very different performance. I believe Thorsten was thinking out loud there!

One reason the current source filament drive sounds good is that the dynamic impedance (presented to the filament) is high. Adding a voltage regulating loop results in LOW dynamic impedance, and the sound is degraded.

If you are concerned with ease of build, you could use the LT108x as current sink (negative end) but try to build the gyrator - it's easy to do, with no stability troubles.

Alternatively, could even use a FET for a single transistor gyrator solution. In that case, use 1M gate supply resistor, 470nF capacitor, 1K gate resistor stopper (right at the gate pin).
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Old 9th November 2009, 05:18 PM   #119
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Quote:
Originally Posted by CFT View Post
Great Rod, so that nice circuit orginated from you!

I gathered at the end of that post, Thorsten suggested something like using the LT1085/1033 pair.

Could anyone come up with that circuit? Would it simply be that just using 1085 ccs in the + path to filament while 1033 ccs put in place at the - path? So a current source and a current sink is born?
Cheung, try this for FET-gyrator version. Just make sure to us a FET with high power handling, and low threshold [Vgs(th)]. STP55NF06L seems a good low cost starting point.

I prefer the bipolar transistor current source, but for folks that like the chips, here's the LT1084 in this case. You might get away with 5.1V - but 5,5V supply would probably be better. The FETs have higher overhead than bipolar transistors (and so will get hotter, too). Take care to bolt the FET to some big piece of chassis!
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Old 9th November 2009, 07:45 PM   #120
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Hi Rod,

Can you explain your circuit for the electronically faint hearted. I'm only used to the LM1084 with adjust to the positive voltage, and I don't understand why R2 isn't 1.2R. I don't understand R1 at all!

Doing well, eh!

andy
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